1. Field of the Invention
This invention relates to an intake system for an engine, and more particularly to an intake system for an engine which is arranged for supercharging the engine by the kinetic effect of intake air.
2. Description of the Prior Art
Various multiple-cylinder engines are known which are arranged to increase the charging efficiency, and thereby the engine output torque, by the kinetic effect of intake air such as an inertia effect or a resonance effect of intake air. For example, in the multiple-cylinder engine disclosed in Japanese Unexamined Patent Publication No. 62(1987)-121828, the cylinders are divided into two groups so that the cylinders in each group do not fire one after another, discrete passages for the cylinders in each group are connected to an enlarged volume chamber, the enlarged volume chambers for the respective cylinder groups are connected to the downstream ends of a pair of long resonance passages which are connected to each other at their upstream ends, and the enlarged volume chambers are connected by a short resonance passage which is provided with an on-off valve.
In such an engine, supercharging effect by the kinetic effect of intake air can be obtained over a wide engine speed range by opening and closing the on-off valve according to the operating condition of the engine. For example, when the on-off valve is closed in a low engine speed range, an air column between the intake port for each cylinder and the upstream end of the long resonance passage is oscillated by a negative pressure wave generated in the intake stroke of the cylinder and when the engine speed tunes to the natural frequency of the air column, the amplitude of the oscillation of the air column is maximized and a resonance supercharging effect is obtained, whereby the charging efficiency is increased. When the on-off valve is opened in a high engine speed range, an air column between the intake port for each cylinder and the enlarged volume chamber for the other cylinder group with which the cylinder is communicated by way of the short resonance passage is oscillated by said negative pressure wave and when the engine speed tunes to the natural frequency of the air column, the amplitude of the oscillation of the air column is maximized and another resonance supercharging effect is obtained, whereby the charging efficiency is increased. When the engine speed increases further higher, a negative pressure generated at the beginning of the intake stroke of each cylinder is reflected as a positive pressure wave at the enlarged volume chamber and propagates downstream and acts on the same cylinder at the end of the intake stroke to supercharge the cylinder by an inertia effect of the intake air. The inertia supercharging effect is maximized at a particular engine speed which is determined mainly according to each discrete passage.
Generally the resonance supercharging effect and the inertia supercharging effect are not independent of each other but have a close relation to each other by way of, for instance, the passage length of the intake system as a parameter. That is, when an inertia supercharging effect is obtained at a certain engine speed, a resonance supercharging effect corresponding to the inertia supercharging effect is obtained at a particular engine speed lower than the certain engine speed.
On the other hand, it has been known that a high positive pressure is produced and an excellent resonance supercharging effect can be obtained when the engine speed is near the resonance tuning point while as the engine speed moves away from the resonance tuning point, a negative pressure sharply increases. This tendency is enhanced as the resonance tuning engine speed increases. Especially when the inertia tuning point is set near the maximum allowable engine speed, the resonance supercharging effect which is obtained at an engine speed lower than the upper limit adversely affects the inertia supercharging effect which is to be obtained near the upper limit and the engine output power cannot be satisfactorily increased by the inertia supercharging effect near the maximum allowable engine speed.